is it possible to make an animation / render of just the glossy passes?
Here how to in Cycles:


First, enable Glossy passes in Render Layers:





Next open node editor in Compositing mode and click Use nodes





Here is the glossy passes, all of them are important.





To make glossy pass you should combine this 3 passes in specific way.
Add 2 Color mix nodes (Shift+A -> Color -> Mix) Change one of them to Add, second to Multiply. Gloss Direct will add to Gloss Indirect, then this sum will multiply by Gloss Color (Connect nodes in this way):


  


So, in this point you can start to render